A transmission in a vehicle downshifts to a lower gear in response to various vehicle or driver behavior. For example, the transmission may downshift to a lower gear when the driver increases the throttle by a particular degree. In this instance, a throttle increase indicates a desire to accelerate the vehicle. A decrease in the speed of the vehicle may result in a coast downshift. During a coast downshift, the transmission detects that the vehicle has slowed below a certain threshold for the current gear and downshifts to a lower gear. Additionally, a manual downshift initiated by the driver causes the transmission to downshift to a lower gear. A driver may initiate a manual downshift to improve engine braking.
During a transmission downshift, the transmission turbine must accelerate to a speed level that is appropriate for the target gear. The transmission turbine spins at the same speed as the input of the transmission and determines how quickly the transmission is able to shift from one gear to another. Therefore, determining the appropriate acceleration of the turbine is important in order to establish effective downshifts. Various vehicle and environment conditions may affect the ability of the transmission turbine to accelerate properly. For example, the altitude of the vehicle affects air pressure and transmission performance. Current vehicle speed, transmission turbine speed, and torque converter slip also affect the appropriate turbine acceleration.